There are many HTML forms used on the World Wide Web (WWW). HTML forms require users who want access to the content behind the form to fill in or select values for one or more different input fields in the form and make a submission. The pages resulting from such submissions can be very useful to web users. The content that lies hidden behind such forms is enormous by some estimates. This notion is often referred to by the terms Deep Web, Hidden Web or Invisible Web.
However, result pages obtained from a form page may not be indexed by search engines if the web-crawler does not have the ability to automatically fill out HTML forms. As such, the web crawler may not be able to detect the hidden pages. This presents a gap in the coverage of search engines (and hence the names Hidden, Deep or Invisible Web for such content).
An approach of creating URLs corresponding to all possible combinations of input-values can provide either or both of the following results. First, no valid HTML page may exist for the URL created by appending input-name-value pairs to the form action. Second, because there may be numerous possible combinations of input values for the different input fields, it is possible that a very large number of URLs must be created (corresponding to each submission of a combination of input values). For example, cars.com has an inventory of about 50,000 cars, but the number of possible form submissions for their search page on www.cars.com is more than a million.